Abstract

The recent identification of Holocene volcanic clusters in small areas within the Michoacan Guanajuato Volcanic Field (MGVF) opens several questions regarding future volcanic hazard assessments in this region. Documenting vent alignments and eruption recurrence intervals within clusters will provide parameters necessary for making temporal and spatial hazard evaluations. Here, we present a possible new case of a small cluster consisting of only two monogenetic volcanoes, El Astillero and El Pedregal located in the similar to 4400-km(2) Tancitaro-Nueva Italia region in the southwestern part of the MGVF, only 25 km to the south of Paricutin volcano. We determined from paleomagnetic and radiocarbon dating that El Astillero and El Pedregal most likely erupted one after the other between AD 500 and 700 (within the Late Classic period of Mesoamerican archeology). While the eruptions were likely separated by a short period of time, the exact length is difficult to ascertain. After the similar to 6 years of total estimated eruption duration of the two volcanoes, both together occupied an area of 14.7 km(2) and emitted a dense rock equivalent (DRE) volume of magma of similar to 0.5 km(3). Notable characteristics of the eruptions include a switch from the explosive activity exclusive of El Astillero (Strombolian) to effusive activity early after the initiation of the El Astillero eruption, a shift in the active vents, and a progressive change in the bulk magma composition from basaltic andesite to andesite throughout the duration of the eruption. This activity first formed the El Astillero scoria cone and tephra deposits followed by its lava field and ended with the emplacement of the El Pedregal viscous lavas. The discovery of pre-Hispanic pottery sherds and obsidian artifacts underneath the El Astillero tephra fallout unambiguously attests to human activities in the area before the eruption. Judging by their eruptive style, the eruptions probably had a limited impact on the small area affected and the surrounding human activities, but the hazard for this area remains since El Astillero and El Pedregal could represent the initial stages of a new cluster that is still in the making. If so, another eruption should be expected in this area again.